Layer transfusion with part heating for additive manufacturing

ABSTRACT

An additive manufacturing system comprising a transfer medium configured to receive the layers from a imaging engine, a heater configured to heat the layers on the transfer medium, and a layer transfusion assembly that includes a build platform, and is configured to transfuse the heated layers onto the build platform in a layer-by-layer manner to print a three-dimensional part.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to U.S. Provisional ApplicationNo. 61/538,491, filed on Sep. 23, 2011, and entitled “LAYER TRANSFUSIONFOR ELECTROPHOTOGRAPHY-BASED ADDITIVE MANUFACTURING”.

Reference is hereby made to co-filed U.S. patent application Ser. No.13/324,495, entitled “LAYER TRANSFUSION FOR ADDITIVE MANUFACTURING”.

Reference is also hereby made to co-filed U.S. patent application Ser.No. 13/624,507, entitled “LAYER TRANSFUSION WITH TRANSFIXING FORADDITIVE MANUFACTURING”.

Reference is also hereby made to co-filed U.S. patent application Ser.No. 13/624,519, entitled “LAYER TRANSFUSION WITH ROTATABLE BELT FORADDITIVE MANUFACTURING”.

BACKGROUND

The present disclosure relates to additive manufacturing systems forbuilding three-dimensional (3D) parts and support structures. Inparticular, the present disclosure relates to additive manufacturingsystems and processes for building 3D parts and support structures usingan imaging process, such as electrophotography.

Additive manufacturing systems are used to build 3D parts from digitalrepresentations of the 3D parts (e.g., STL format files) using one ormore additive manufacturing techniques. Examples of commerciallyavailable additive manufacturing techniques include extrusion-basedtechniques, ink jetting, selective laser sintering, powder/binderjetting, electron-beam melting, and stereolithographic processes. Foreach of these techniques, the digital representation of the 3D part isinitially sliced into multiple horizontal layers. For each sliced layer,a tool path is then generated, which provides instructions for theparticular additive manufacturing system to form the given layer.

For example, in an extrusion-based additive manufacturing system, a 3Dpart or model may be printed from a digital representation of the 3Dpart in a layer-by-layer manner by extruding a flowable part material.The part material is extruded through an extrusion tip carried by aprint head of the system, and is deposited as a sequence of roads on asubstrate in an x-y plane. The extruded part material fuses topreviously deposited part material, and solidifies upon a drop intemperature. The position of the print head relative to the substrate isthen incremented along a z-axis (perpendicular to the x-y plane), andthe process is then repeated to form a 3D part resembling the digitalrepresentation.

In fabricating 3D parts by depositing layers of a part material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of objects under construction, whichare not supported by the part material itself. A support structure maybe built utilizing the same deposition techniques by which the partmaterial is deposited. The host computer generates additional geometryacting as a support structure for the overhanging or free-space segmentsof the 3D part being formed. Support material is then deposited from asecond nozzle pursuant to the generated geometry during the printingprocess. The support material adheres to the modeling material duringfabrication, and is removable from the completed 3D part when theprinting process is complete.

In two-dimensional (2D) printing, electrophotography (i.e., xerography)is a popular technology for creating 2D images on planar substrates,such as printing paper. Electrophotography systems include a conductivesupport drum coated with a photoconductive material layer, where latentelectrostatic images are formed by charging and then image-wise exposingthe photoconductive layer by an optical source. The latent electrostaticimages are then moved to a developing station where toner is applied tocharged areas of the photoconductive insulator to form visible images.The formed toner images are then transferred to substrates (e.g.,printing paper) and affixed to the substrates with heat or pressure.

SUMMARY

An aspect of the present disclosure is directed to an additivemanufacturing system for printing a thermoplastic part. The additivemanufacturing system includes an imaging engine configured to developimaged layers of a thermoplastic-based powder having a fusiontemperature, a movable build platform, and a transfer medium having atransfer surface configured to sequentially receive and convey theimaged layers from the imaging engine to the build platform, and havingan opposing contact surface. The system also includes a heaterconfigured to heat the imaged layers on the transfer medium to at leastthe fusion temperature, and a heater configured to heat a previouslytransfused layer of a thermoplastic part being printed on the buildplatform to at least the fusion temperature. The system further includesa heated layer transfusion element configured to transfuse a heatedimaged layer conveyed by the transfer medium onto the heated previouslyfused layer by engaging the contact surface of the transfer medium sothat the transfer medium and the heated imaged layer are pressed betweenthe transfuse element and the build platform, and to disengage therefromwithout releasing the transfer medium from the transfused layer. Thesystem further includes a cooling unit configured to actively cool thetransfused layer to below the fusion temperature while it remains on thetransfer medium, so as to transfix the transfused layer beforedelaminating it from the transfer medium.

Another aspect of the present disclosure is directed to an additivemanufacturing system for printing a thermoplastic part, which includesan imaging engine configured to develop imaged layers of athermoplastic-based powder having a fusion temperature, a movable buildplatform, and a transfer medium having a transfer surface configured tosequentially receive and convey the imaged layers from the imagingengine to the build platform, and having an opposing contact surface.The system also includes a heater configured to heat the imaged layerson the transfer medium to at least the fusion temperature, and a heaterconfigured to heat a previously transfused layer of a thermoplastic partbeing printed on the build platform to at least the fusion temperature.The system further includes a heated fusion roller configured to beheated to a transfer temperature and further configured to transfuse aheated imaged layer conveyed by the transfer medium onto the heatedpreviously fused layer by engaging and rolling across the contactsurface of the transfer medium, and to disengage therefrom withoutreleasing the transfer medium from the transfused layer. The systemfurther includes a cooling unit configured to actively cool thetransfused layer to below the fusion temperature while it remains on thetransfer medium so as to transfix the transfused layer, and a releaseroller disposed downstream of the cooling unit and configured to engagethe contact surface of the transfer medium so as to assist indelaminating the transfused layer from the transfer medium.

Another aspect of the present disclosure is directed to a method forprinting a 3D part with an additive manufacturing system. The methodincludes imaging a layer of the 3D part from a thermoplastic-basedpowder, transferring the imaged layer to a transfer medium, heating theimaged layer while the imaged layer is retained on the transfer medium,and heating a top surface of the 3D part. The method also includestransfusing the heated layer to the heated top surface of the 3D part,cooling the transfer medium and at least a portion of the transfusedlayer, and releasing the transfused layer from the cooled transfermedium such that the transfused layer remains adhered to the 3D part.

In some embodiments, the additive manufacturing system is configured toprint or otherwise produce the layers at a rate that is faster than apassive thermal diffusion rate of the thermoplastic or 3D part.

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The terms “transfusion”, “transfuse”, “transfusing”, and the like referto the adhesion of layers with the use of heat and pressure, wherepolymer molecules of the layers at least partially interdiffuse.

The term “transfusion pressure” refers to a pressure applied during atransfusion step, such as when transfusing layers of a 3D part together.

The term “deformation temperature” of a 3D part refers to a temperatureat which the 3D part softens enough such that a subsequently-appliedtransfusion pressure, such as during a subsequent transfusion step,overcomes the structural integrity of the 3D part, thereby deforming the3D part.

Unless otherwise specified, temperatures referred to herein are based onatmospheric pressure (i.e. one atmosphere).

Directional orientations such as “above”, “below”, “top”, “bottom”, andthe like are made with reference to a direction along a printing axis ofa 3D part. In the embodiments in which the printing axis is a verticalz-axis, the layer-printing direction is the upward direction along thevertical z-axis. In these embodiments, the terms “above”, “below”,“top”, “bottom”, and the like are based on the vertical z-axis. However,in embodiments in which the layers of 3D parts are printed along adifferent axis, the terms “above”, “below”, “top”, “bottom”, and thelike are relative to the given axis.

The term “providing”, such as for “providing a material” and the like,when recited in the claims, is not intended to require any particulardelivery or receipt of the provided item. Rather, the term “providing”is merely used to recite items that will be referred to in subsequentelements of the claim(s), for purposes of clarity and ease ofreadability.

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an electrophotography-basedadditive manufacturing system of the present disclosure having a layertransfusion assembly with a press plate.

FIG. 2 is a top view of a build platform receiving a heated layer,illustrating an air knife arrangement.

FIG. 3 is a graphical illustration of number of printed layers for a 3Dpart versus an average part temperature, showing plot lines for a 3Dpart printed without active cooling and for a 3D part printed withactive cooling.

FIGS. 4A-4D are expanded views of the layer transfusion assembly,illustrating a layer transfer technique of the present disclosure.

FIGS. 5A-5E are expanded views of an alternative layer transfusionassembly of the electrophotography-based additive manufacturing system,having a nip roller, and which illustrate the layer transfer technique.

FIGS. 6A-6F are expanded views of another alternative layer transfusionassembly of the electrophotography-based additive manufacturing system,having separate transfusion and release rollers, and which illustratethe layer transfer technique.

FIG. 7 is an expanded view of another alternative layer transfusionassembly of the electrophotography-based additive manufacturing system,having an enclosable chamber.

FIG. 8 is a sectional side of a rotatable transfer belt.

FIG. 9A is a top view of a first embodiment of the rotatable transferbelt, having receiving regions and encoder markings.

FIG. 9B is a top view of a second embodiment of the rotatable transferbelt, having holes for engagement in a tractor-feed manner.

FIG. 9C is a bottom view of a third embodiment of the rotatable transferbelt, having rear ribs for engagement in a timing-belt manner.

FIG. 10 is a flow diagram of a first embodied method for the layertransfer technique of the present disclosure, having a combinedtransfusion and transfixing step, and an active cooling step.

FIG. 11 is a flow diagram of a second embodied method for the layertransfer technique, having separate transfusion and transfixing steps,and an optional active cooling step.

FIG. 12 is a flow diagram of a third embodied method for the layertransfer technique, having a part surface heating step, separatetransfusion and transfixing steps, and an active cooling step.

DETAILED DESCRIPTION

The present disclosure is directed to a layer transfer technique forprinting 3D parts and support structures in a layer-by-layer manner,where each layer is printed from a part or support material in athermally-controlled manner. The layer transfer technique is performedwith an imaging system, such as an electrophotography-based additivemanufacturing system. For example, each layer may be developed usingelectrophotography and carried from an electrophotography (EP) engine bya transfer medium (e.g., a rotatable belt or drum). The layer is thentransferred to a build platform to print the 3D part (or supportstructure) in a layer-by-layer manner, where the successive layers aretransfused together to produce the 3D part (or support structure).

In comparison to 2D printing, in which developed toner particles can beelectrostatically transferred to printing paper by placing an electricalpotential through the printing paper, the multiple printed layers in a3D environment effectively prevents the electrostatic transfer of partand support materials after a given number of layers are printed (e.g.,about 15 layers). Instead, in the present disclosure, a layer retainedby the transfer medium is heated to at least a fusion temperature of thelayer material. The heated layer is then pressed against apreviously-printed layer (or to a build platform) to transfuse thelayers together (i.e., a transfusion step). This allows numerous layersof 3D parts and support structures to be built vertically, beyond whatis otherwise achievable via electrostatic transfers.

As discussed below, the fusion temperature is a temperature thatsufficiently melts the layer material to a fusable state such thatpolymer molecules of the layer material at least partially interdiffuseduring the subsequent transfusion step to promote interlayer orinterfacial entanglement. The fusion temperature is high enough topromote the layer transfusion, but it can be too hot for the transfusedlayer to cleanly release or otherwise delaminate from the transfermedium. This can potentially result in portions of the transfused layerremaining adhered to the transfer medium, or smear upon release from thetransfer medium, which negatively impacts feature detail, dimensionalaccuracy, and porosity of a printed 3D part.

Accordingly, in some embodiments, the layer transfer technique may alsoinclude a “transfixing step”, in which the transfer medium and/or thetransfused layer is cooled prior to releasing the transfused layer fromthe transfer medium. While not wishing to be bound by theory, it isbelieved that this transfixing step cools down the interface between thetransfer medium and the transfused layer, thus increasing the adhesiveforce of the interdiffused polymers in adjacent layers relative to theadhesive force of the transfused layer to the surface of the transfermedium, thereby maintaining the transfused layer adhered to the 3D partin a fixed spatial position. This allows the transfused layer to cleanlyrelease from the transfer medium and remain adhered to the 3D part.

Furthermore, because the imaging system is capable of printing thelayers at speeds that are much faster than the rate at which heatdiffuses through the variable thermal resistance of the 3D parts, heataccumulation in the 3D parts has been observed. As such, as the heightof a given 3D part grows, heat dissipation from passive thermaldiffusion becomes insufficient to cool the heated layers. The faster thelayer speed, the faster the heat accumulation in the bulk of the 3Dpart. As successive layers are continuously printed, this heataccumulation may exceed a deformation temperature of the 3D part,causing the bulk of the 3D part to soften enough reduces its structuralintegrity. Such a soft part may deform under a subsequently-appliedtransfusion pressure during a subsequent transfusion step.

In some embodiments, heat accumulation can be reduced by slowing downthe printing process. As can be appreciated, this can substantiallyincrease the time required to print 3D parts, thereby reducingthroughput. Instead, to overcome this issue while maintaining fastprinting rates, the layer transfer technique may include an “activecooling step” to prevent the 3D part from accumulating additional heat,thereby maintaining the 3D part at a lower “average part temperature”that is lower than the deformation temperature of the 3D part.

In particular, after each layer of the 3D part is transfused, the heatadded to the 3D part from the transfused layer is substantially removedprior to the transfusion of the next layer. This holds the 3D part at anaverage part temperature that is desirably balanced to promoteinterlayer adhesion and reduce the effects of curling, while also beinglow enough to prevent the 3D part from softening too much (i.e., belowits deformation temperature).

As shown in FIG. 1, system 10 is an example additive manufacturingsystem for printing 3D parts and support structures usingelectrophotography, which incorporates the layer transfer technique ofthe present disclosure. System 10 includes EP engine 12, transfer belt14, rollers 16, build platform 18, and press plate 20 for printing 3Dparts (e.g., 3D part 22) and any associated support structures (notshown). Examples of suitable components and functional operations forsystem 10 include those disclosed in U.S. patent application Ser. Nos.13/242,669 and 13/242,841.

In alternative embodiments, system 10 may include different imagingengines for imaging the layers. As discussed below, the layer transfertechnique focuses on the transfer of layers from belt 14 (or othertransfer medium) to build platform 18, and on belt 14, rather than onthe particular imaging engine. However, the layer transfer technique isparticularly suitable for use with electrophotography-based additivemanufacturing systems (e.g., system 10), where the multiple printedlayers in a 3D environment effectively prevents the electrostatictransfer of part and support materials after a given number of layersare printed, as discussed above.

System 10 also includes controller 24, which is one or more controlcircuits, microprocessor-based engine control systems, and/ordigitally-controlled raster imaging processor systems, and which isconfigured to operate the components of system 10 in a synchronizedmanner based on printing instructions received from host computer 26.Host computer 26 is one or more computer-based systems configured tocommunicate with controller 24 to provide the print instructions (andother operating information). For example, host computer 26 may transferinformation to controller 24 that relates to the sliced layers of 3Dpart 22 (and any support structures), thereby allowing system 10 toprint 3D part 22 in a layer-by-layer manner.

As discussed in the U.S. patent application Ser. Nos. 13/242,669 and13/242,841, EP engine 12 is configured to develop successive layers 28of a thermoplastic-based powder using electrophotography. As usedherein, the term “electrophotography” includes ionography. Thethermoplastic-based powder includes one or more thermoplastic materials(e.g., an acrylonitrile-butadiene-styrene (ABS) copolymer), and may alsoinclude one or more additional components for development with EP engine12 and triboelectric attraction to belt 14.

The developed layers 28 of the thermoplastic-based powder are thenrotated to a first transfer region in which layers 28 are transferredfrom EP engine 12 to belt 14. Belt 14 is an example transfer medium fortransferring or otherwise conveying the developed layers 28 from EPengine 12 to build platform 18 with the assistance of press plate 20. Inthe shown embodiment, belt 14 includes front or transfer surface 14 aand rear or contact surface 14 b, where front surface 14 a faces EPengine 12. As discussed below, in some embodiments, belt 14 may be amultiple-layer belt with a low-surface-energy film that defines frontsurface 14 a, and which is disposed over a base portion that definesrear surface 14 b.

System 10 may also include biasing mechanism 29, which is configured toinduce an electrical potential through belt 14 to electrostaticallyattract layers 28 of the thermoplastic-based powder from EP engine 12 tobelt 14. Because layers 28 are each only a single layer increment inthickness at this point in the process, electrostatic attraction issuitable for transferring layers 28 from EP engine 12 to belt 14.However, as mentioned above, the multiple printed layers 28 for 3D part22 effectively prevents electrostatic transfer of layers 28 from belt 14to build platform 18 after a given number of layers 28 are printed.

Rollers 16 are a series of drive and/or idler rollers or pulleys thatare configured to maintain tension on belt 14 while belt 14 rotates inthe rotational direction of arrows 30. This allows belt 14 to maintain asubstantially planar orientation when engaging build platform 18 andpress plate 20. System 10 may also include service loops (not shown),such as those disclosed in U.S. patent application Ser. No. 13/242,841.

System 10 also includes heater 32, located upstream from build platform18 and press plate 20, based on the rotational direction of belt 14.Heater 32 is one or more heating devices configured to heat layers 28 toat least a fusion temperature of the thermoplastic-based powder prior toreaching press plate 20. Examples of suitable devices for heater 32include non-contact radiant heaters (e.g., infrared heaters or microwaveheaters), convection heating devices (e.g., heated air blowers), contactheating devices (e.g., heated rollers and/or platens), combinationsthereof, and the like, where non-contact radiant heaters are preferred.Each layer 28 desirably passes by (or through) heater 32 for asufficient residence time to heat the layer 28 to the intended fusiontemperature.

As defined above, the fusion temperature is a temperature thatsufficiently melts the thermoplastic-based powder to a fusable state.Thus, the fusion temperature will vary depending on the particular layermaterial used. For example, for an ABS copolymer material, the fusiontemperature may range from about 180° C. to about 220° C. depending onthe particular copolymer composition. Heating the thermoplastic-basedpowder does not necessarily require every component of thethermoplastic-based powder to melt. Rather, the overallthermoplastic-based powder needs to reach a fusable state for subsequenttransfusion. This typically refers to the one or more thermoplasticmaterials of the thermoplastic-based powder being sufficiently melted tothe fusable state.

Build platform 18, press plate 20, and heater 32 may collectively bereferred to as layer transfusion assembly 33. Layer transfusion assembly33 is configured to transfuse the heated layers 28 from the belt 14 tothe previously-transfused layers of 3D part 22 (or onto build platform18) in a layer-by-layer manner.

Build platform 18 is a platform assembly or platen of system 10 that isconfigured to receive the heated layers 28 for printing 3D part 22 in alayer-by-layer manner. Build platform 18 is supported by z-axis gantry34, which is a linear guide mechanism configured to incrementally lowerbuild platform 18 along the vertical z-axis relative to belt 14 andpress plate 20 after each pressing step. The movement of build platform18 with z-axis gantry 34 is operated by z-axis motor 36 based oncommands from controller 24, where z-axis motor 36 may be an electricalmotor, a hydraulic system, a pneumatic system, or the like.

In the shown embodiment, build platform 18 is heatable with heatingelement 38 (e.g., an electric heater). Heating element 38 is configuredto heat and maintain build platform 18 at an elevated temperature thatis greater than room temperature (25° C.), such as at the desiredaverage part temperature of 3D part 22. This allows build platform 18 toassist in maintaining 3D part 22 at this average part temperature.

As mentioned above, the average part temperature for 3D part 22 isdesirably high enough to promote interlayer adhesion and to reduce theeffects of curling, while also being low enough to prevent 3D part 22from softening too much (i.e., below its deformation temperature).Suitable average part temperatures for 3D part 22 range from greaterthan the average solidification temperature of the thermoplasticmaterial(s) of the thermoplastic-based powder to about the glasstransition temperature of the thermoplastic material(s). More desirably,the average part temperature is maintained at about the creep relaxationtemperature of the thermoplastic material(s) of the thermoplastic-basedpowder, or within about 10° C. above or below thereof. Examples ofsuitable techniques for determining the creep relaxation temperatures ofmaterials are disclosed in Batchelder et al., U.S. Pat. No. 5,866,058.

For example, when printing layers 28 of an ABS copolymer-based powder,the average part temperature for 3D part 22 may be about 100° C. Thisaverage part temperature allows 3D part 22 to maintain its structuralintegrity when pressed between build platform 18 and press plate 20during subsequent transfusion steps. Furthermore, when the top-mostlayer of 3D part 22 maintained at this temperature and receives a heatedlayer 28 at a fusion temperature of about 200° C., the transfusioninterface temperature for transfusing the layers together starts atabout 150° C. This temperature is suitable for the polymer molecules ofthe layers 28 to at least partially interdiffuse to promote interfacialentanglement.

Press plate 20 is an example heateable element or heatable layertransfusion element, which is configured to press belt 14 and a givenheated layer 28 downward against 3D part 22 and build platform 18 duringeach transfusion step. Press plate 20 may be actuated with a servomechanism (not shown) configured to move press plate 20 along thevertical z-axis during each transfusion step.

The particular pressure applied during each transfusion step isdesirably high enough to adhere the heated layer 28 to thepreviously-transfused layer 28 (or to build platform 18), allowing thepolymer molecules to at least partially interdiffuse. However, thetransfusion pressure is also desirably balanced to prevent press plate20 from compressing 3D part 22 too much, thereby allowing 3D part 22 tomaintain its dimensional integrity.

In the shown embodiment, press plate 20 is heatable with heating element40 (e.g., an electric heater). Heating element 40 is configured to heatand maintain press plate 20 at an elevated temperature that is greaterthan room temperature (i.e., 25° C.). However, in comparison to theelevated temperature of build platform 18, heating element 40 may heatand maintain press plate 20 at a temperature that is less than thedesired average part temperature of 3D part 22. For example, insituations where the intended average part temperature for 3D part 22 is100° C., heating element 40 may heat and maintain press plate 20 atabout 70° C.

The lower temperature for press plate 20 will cool belt 14 from the sideof rear surface 14 b (i.e., a transfixing step). As discussed above, ifthe transfused layer 28 remains too hot, it may remain adhered to frontsurface 14 a of belt 14, rather than cleaning releasing from belt 14. Assuch, cooling belt 14 with the contact from press plate 20 allows theinterface between front surface 14 a of belt 14 and the transfused layer28 to cool down enough to cleanly release the transfused layer 28 frombelt 14.

On the other hand, due to the large contact surface area between belt 14and press plate 20, if press plate 20 is maintained at a temperaturethat is too low (e.g., 25° C.), the contact duration between press plate20 and belt 14 during the transfusion step may cool the transfused layer28 down too much, thereby lowering the transfusion interfacetemperature, which can reduce the interlayer adhesion. As such, in someembodiments, heating element 40 desirably maintains press plate 20 at atemperature that balances these competing thermal effects to facilitateboth transfusion and transfixing in a single, combined step.

System 10 may also include one or more air knives 42 or other coolingunits, where air knife 42 is an example cooling unit configured to blowlocalized cooling air to the top layers of 3D part 22. As best shown inFIG. 2, air knife 42 is located adjacent to the lateral side of buildplatform 18 to direct the cooling air laterally relative to thedirection of movement of belt 14. This allows air knife 42 to extendalong the entire length of 3D part 22, providing good air flow over thetop layers of 3D part 22, including the fused layer 28. In analternative embodiment, system 10 may also include a second air knife 42(not shown) located at the opposing lateral side of build platform 18.In embodiments having air knives 42 or other cooling units, press plate20 may be heated to the fusion temperature to promote interdiffusion inthe transfusion step. Then, upon release of press plate 20, a separatetransfix step may be performed by cooling the transfused layer beforerelease of the layer from the belt 14.

As mentioned above, when system 10 prints layers 28 at high speeds, theprinted layers 28 do not have sufficient time to cool down to thedesired average part temperature before successive layers 28 areprinted. As such, as the height of 3D part 22 grows, heat dissipationfrom passive thermal diffusion becomes insufficient to cool the heatedlayers.

This is depicted in FIG. 3, which is a graphical illustration of thenumber of layers printed versus the average part temperature for a 3Dpart printed without the active cooling (represented by line 44) and for3D part 22 printed with the active cooling (represented by line 46). Asshown by line 44, without the active cooling, the heat added by eachlayer at its fusion temperature will accumulate in the 3D part, causingthe average part temperature to increase until the deformationtemperature of the 3D part is reached, as illustrated by threshold line48. At threshold line 48, the temperature in the bulk of the 3D part ishigh enough such that the part material substantially softens. When the3D part reaches this point, the transfusion pressure applied by pressplate 20 during subsequent transfusion steps may overcome the structuralintegrity of the 3D part, thereby deforming the 3D part.

Air knife 42, however, actively cools each layer after the transfusionstep to prevent the additional heat from accumulating. As shown by line46, the active cooling substantially removes the heat provided by eachlayer 28, thereby providing substantially zero heat accumulation aftereach printed layer 28. As such, 3D part 22 may be substantiallymaintained at an average part temperature that is below its deformationtemperature during the entire printing operation.

In some embodiments, it can be beneficial for the average parttemperature to be high enough such that the bulk of 3D part 22 exhibitsa small amount of softening. It has been found that when the bulk of 3Dpart 22 exhibits a small amount of softening that still maintains itsoverall structural integrity, subsequent transfusion steps with pressplate 20 may mildly compress the bulk of 3D part 22, thereby increasingthe part density. The increased part density correspondingly reducesbrittleness and porosity of the resulting 3D part 22, and increases itsz-axis strength. These properties are beneficial for a variety ofapplications.

While illustrated with air knife 42, system 10 may alternatively includea variety of different cooling units configured to actively cool eachlayer after the transfusion step to prevent the additional heat fromaccumulating, such as refrigeration units, liquid-cooling units, and thelike. Furthermore, one or more air knives 42 (or other cooling units)may be located at other locations around build platform 18 and pressplate 20 to direct the cooling air towards the top layers of 3D part 22.Optionally, system 10 may also include additional heaters (e.g., heaters270 and 272, shown in FIGS. 6A-6F) to heat the top layer(s) or surfaceof 3D part 22 to the fusion temperature prior to each subsequenttransfusion step.

FIG. 4A-4D illustrate an example process for printing a layer 28 withsystem 10 using the layer transfer technique of the present disclosure.At the start of the printing operation, build platform 18 and pressplate 20 may be heated to their desired temperatures. For example, buildplatform 18 may be heated to the desired average part temperature for 3Dpart 22, and press plate 20 may be heated to a temperature that is lowerthan the desired average part temperature for 3D part 22.

The printing operation initially involves developing a layer 28 with EPengine 12 (shown in FIG. 1) and transferring the developed layer toheater 32 via belt 14. As shown in FIG. 4A, as the developed layer 28passes by (or through) heater 32, heater 32 heats the layer 28 and theassociated region of belt 14 to at least the fusion temperature of thethermoplastic-based powder.

As shown in FIG. 4B, the continued rotation of belt 14 in the directionof arrow 30 aligns the heated layer 28 above build platform 18 withproper registration location along the x-axis. Press plate 20 may thenactuate downward, as illustrated by arrow 50, to press the heated layer28 onto the previously-printed layer of 3D part 22. As shown in FIG. 4C,because layer 28 is heated to at least the fusion temperature of thepart material, the pressed layer 28 transfuses to the top surface of 3Dpart 22.

Examples of suitable durations for pressing during the transfusion steprange from about 0.1 seconds to about 1.0 second, with particularlysuitable durations ranging from about 0.1 seconds to about 0.5 seconds.In some embodiments, the pressing duration is a fixed value for eachlayer 28. In alternative embodiments, the pressing duration may bevaried based on the dimensions and geometry of 3D part 22. For example,the pressing duration may be reduced for layers 28 having smallercross-sectional areas and/or fine-feature details, as discussed below.

The temperature of press plate 20, being lower than the desired averagepart temperature, and substantially lower than the fusion temperature,begins to draw heat from the heated region of belt 14. This assists inreleasing the transfused layer 28 by cooling belt 14 from rear side 14b, as discussed above.

After the transfusion/transfixing step is completed, press plate 20 maythen be retracted upward, as illustrated by arrow 52, to release thepressure applied to belt 14 and the transfused layer 28. In theembodiment shown in FIG. 4D, this releases the transfused layer 28 frombelt 14, allowing the transfused layer 28 to remain transfused to 3Dpart 22. Additionally, z-axis gantry 34 may lower build platform 18downward, such as by a single layer increment, as illustrated by arrow54.

In an alternate embodiment, press plate 20 may be heated to the fusiontemperature to assist in the transfusion of layer 28. In thisembodiment, layer transfusion assembly 33 is desirably configured suchthat retracting press plate 20 upward does not immediately release thetransfused layer 28 from belt 14. Rather, belt 14 may be maintained in arelatively constant position upon the retraction of press plate 20 bypositioning build platform 18 in a higher position relative to thatshown in FIG. 4D during the transfusion step. A separate transfixingstep may then be utilized.

In this transfixing step, air knife 42 may be activated to cool thetransfused layer 28 before releasing it from belt 14. After a durationsufficient to cool the layer material down below the fusion temperature,which maintains the transfused layer 28 in a fixed spatial position andadhered to 3D part 22, build platform 18 is then lowered to releasetransfused layer 28 from belt 14.

Air knife 42 may also be activated to blow cooling air onto thetransfused layer 28 after delamination from belt 14. This actively coolsthe transfused layer 28 down to the desired average part temperature.Examples of suitable durations for this active cooling step range fromabout 1.0 second to about 2.0 seconds, which may correspond with thetransfer and alignment of the next layer 28.

As can be appreciated, the transfer of layer 28 from belt 14 to buildplatform 18 requires a pause during the pressing step. Otherwise, themovement of belt 14 in the rotational direction of arrows 30 during thetransfusion step may cause a mis-registration of the pressed layer 28,potentially resulting in lower part quality. These pauses during eachtransfusion step may be accommodated with the use of service loops, suchas those disclosed in U.S. patent application Ser. No. 13/242,841. Asfurther shown in FIGS. 4B-4D, as a present layer 28 is being transfused,the next layer 28 a may be positioned at heater 32. This allows heater32 to sufficiently heat the next layer 28 a during the required pausewhile the present layer 28 is transfused to 3D part 22. After the layer28 is transfused and cooled, the same process may then be repeated forlayer 28 a, and each subsequent layer for 3D part 22.

FIGS. 5A-5E illustrate layer transfusion assembly 133, which is analternative to layer transfusion assembly 33 of system 10 (shown inFIGS. 1, 2, and 4A-4D), and where the reference numbers of therespective components are increased by “100” from system 10. As shown inFIG. 5A, layer transfusion assembly 133 includes nip roller 120 in lieuof press plate 20, where nip roller 120 is another example heateableelement or heatable layer transfusion element, and is configured torotate around a fixed axis with the movement of belt 114. In particular,nip roller 120 may roll against rear surface 114 b in the direction ofarrow 156 while belt 114 rotates in the direction of arrows 130. In someembodiments, nip roller 120 may function as a drive roller for belt 114.

As further shown, air jets 142 (or other suitable cooling units) areused in lieu of air knife 42, and are located downstream from theinterface between belt 114 and nip roller 120. Air jets are configuredto blow cooling air towards the top layers of 3D part 122 to activelycool the layers after each transfusion step, as discussed below.

In this embodiment, build platform 118 is supported by gantry 134, whichis a guide mechanism configured to move build platform 118 along thez-axis and the x-axis to produce a reciprocating rectangular pattern,where the primary motion is back-and-forth along the x-axis. Gantry 134may be operated by motor 136 based on commands from controller 124,where motor 136 may be an electrical motor, a hydraulic system, apneumatic system, or the like. Accordingly, the pressure that is appliedduring each transfusion step is performed by build platform 118.

Prior to printing 3D part 122, build platform 118 and nip roller 120 maybe heated to their desired temperatures, as discussed above for buildplatform 18 and press plate 20. In comparison to press plate 20, heatingelement 140 may heat nip roller 120 to a higher temperature (e.g., tothe average part temperature) since there is a relatively short time forthe heat to locally flow from nip roller 120 through belt 114.

During the printing operation, heater 132 heats a developed layer 128and the associated region of belt 114 to at least the fusion temperatureof the thermoplastic-based powder. Belt 114 may then move the heatedlayer 132 to a predetermined registration location along the x-axis, asshown. Gantry 134 may then actuate build platform 118 upward to engagebelt 114, which presses the top layer of 3D part 122 against the heatedlayer 124, as illustrated by arrow 158. Alternatively, nip roller 120may be actuated downward to meet the top layer or surface of 3D part122.

As shown in FIG. 5B, this presses the heated layer 128 between the toplayer of 3D part 122 and belt 114 at the location of nip roller 120.While build platform 118 remains engaged with belt 114, gantry 134 maythen move build platform 118 (and 3D part 122) along the x-axis in thedirection of arrow 160, at a rate that is synchronized with therotational rate of belt 114 in the direction of arrow 130 (i.e., thesame directions and speed). This presses belt 114 and the heated layer128 between the top layer of 3D part 122 and nip roller 120.

As shown in FIG. 5C, this causes rear surface 114 b of belt 114 to rollacross nip roller 120 to nip belt 114 and the heated layer 128 againstthe top layer of 3D part 122. Because layer 128 is heated to the fusiontemperature of the part material and 3D part 122 is maintained at theaverage part temperature, the pressed layer 128 transfuses to the toplayer of 3D part 122 in a similar manner to that discussed above for 3Dpart 22 and layer 28.

As further shown, as the transfused layer 128 passes the nip of niproller 120, belt 114 wraps around nip roller 120 to separate anddisengage from build platform 118. This assists in releasing thetransfused layer 128 from belt 114, allowing the transfused layer 128 toremain adhered to 3D part 122. Additionally, air jets 142 blow coolingair towards the top layers of 3D part 122 as build platform 118 movesalong the x-axis past nip roller 120 to transfix the transfused layer128. The transfused layer 128 may be cooled down to the average parttemperature by continuing to blow cooling air against the 3D part 122 inbetween transfusion steps, thus overlapping the transfixing and activecooling steps.

When build platform 118 moves 3D part 122 past nip roller 120, gantry134 may actuate build platform 118 (and 3D part 122) downward, asillustrated by arrow 162. As shown in FIG. 5D, gantry 134 may then movebuild platform 118 (and 3D part 122) along the x-axis in the directionof arrow 164, back to a starting position along the x-axis. As shown inFIG. 5E, build platform 118 desirably reaches the starting position asthe next heated layer 128 is positioned above 3D part 122. The sameprocess may then be repeated, where gantry 134 actuates build platform118 upward to press belt 114 and the next heated layer 128 between thetop layer of 3D part 122 and roller 120, as illustrated by arrow 166. Inthis step, however, the height of the top surface of 3D part 122 isoffset downward, such as by a single layer increment, compared to theprevious pressing step.

As can be appreciated, moving build platform 118 (and 3D part 122) inthe reciprocating rectangular pattern allows the transfusion steps to beperformed while belt 114 continuously rotates. In particular, movingbuild platform 118 at a rate that is synchronized with the rotationalrate of belt 114, along with the use of nip roller 120, which rotatesagainst rear surface 114 b of belt 114, allows the transfusion andtransfixing steps to be performed rapidly (e.g., within about 0.1 toabout 0.5 seconds). This allows the active cooling steps to range fromabout 1.0 second to about 2.0 seconds, which may correspond with thetransfer and alignment of the next heated layer 28. While thereciprocating rectangular pattern is described as a rectangular patternwith sharp axial corners (defined by arrows 160, 162, 164, and 166),gantry 134 may move build platform 118 (and 3D part 122) in areciprocating rectangular pattern having rounded or oval-definingcorners, so long as build platform 118 moves linearly along the x-axisduring the transfusion steps.

FIGS. 6A-6F illustrate layer transfusion assembly 233, which is analternative to layer transfusion assembly 133 (shown in FIGS. 5A-5E),and where the reference numbers of the respective components areincreased by “200” from system 10 (shown in FIGS. 1, 2, and 4A-4D).Layer transfusion assembly 233 may function in a similar manner to layertransfusion assembly 133, where build platform 218 may move in areciprocating rectangular pattern.

However, as shown in FIG. 6A, layer transfusion assembly 233 includesfusion roller 220 and release roller 268 in lieu of a single nip roller120, where fusion roller 220 and release roller 268 are each configuredto rotate around an axis with the movement of belt 214. The use ofseparate rollers (i.e., fusion roller 220 and release roller 268)separates the functions of each roller, allowing them to be optimizedfor their particular purposes. For example, fusion roller 220 may beheated to the fusion temperature of the thermoplastic-based powder, andrelease roller 268 may be maintained at a substantially lowertemperature to assist in delaminating the transfused layers 228 frombelt 214.

As further shown, system 210 also includes heaters 270 and 272 and airjets 274. The separation of fusion roller 220 and release roller 268creates separate transfusion and release steps, and allows a transfixingstep to be performed therebetween via cooling by air jets 274. Bypostponing the release step, fusion roller 220 may be heated to thefusion temperature best suited for the transfusion step, rather than acompromise temperature that facilitates both transfusion and release.This increases the interlayer adhesion between the transfused layers228.

Prior to printing 3D part 222, build platform 218 and fusion roller 220may be heated to their desired temperatures. For example, build platform218 may be heated to the average part temperature and fusion roller 220may be heated to the fusion temperature of the thermoplastic-basedpowder. During the printing operation, belt 214 carries a developedlayer 228 past heater 232, which heats the developed layer 228 and theassociated region of belt 214 to at least the fusion temperature of thethermoplastic-based powder.

Additionally, platen gantry 234 moves build platform 218 along thex-axis in the direction of arrow 276 below, along, or through heater270. Heater 270 may function in the same manner as heaters 32 and 232,and heats the top surface of 3D part 222 to an elevated temperature,such as at the fusion temperature of the layer material.

As shown in FIG. 6B, the continued rotation of belt 214 and the movementof build platform 218 align the heated layer 228 with the heated topsurface of 3D part 222 with proper registration along the x-axis.Furthermore, the heated layer 228 and the heated top surface of 3D part222 each pass heater 272, which may be configured to heat and/ormaintain both the heated layer 228 and the heated top surface of 3D part222 at the fusion temperature of the layer material. This prevents theheated layer 228 from cooling down prior to reaching fusion roller 220,and brings the temperature of the heated top surface of 3D part 222 toor near the fusion temperature before the next transfusion step isperformed. In alternative embodiments, one or more of heaters 232, 270,and 272 may be provided a single heater configured to direct heat inmultiple directions (e.g., towards both the layer 228 and the topsurface of 3D part 222).

Gantry 234 may continue to move build platform 218 (and 3D part 222)along the x-axis in the direction of arrow 276, at a rate that issynchronized with the rotational rate of belt 214 in the direction ofarrow 230 (i.e., the same directions and speed). This causes rearsurface 214 b of belt 214 to rotate around fusion roller 220 to nip belt214 and the heated layer 228 against the top surface of 3D part 222.This engages build platform 218 and belt 214, and presses the heatedlayer 228 between the heated top surface of 3D part 222 and belt 214 atthe location of fusion roller 220.

Because layer 228 and the heated top layer of 3D part 222 are eachheated to the fusion temperature of the layer material, the pressedheated layer 228 transfuses to the heated top surface of 3D part 222with a high level of interlayer adhesion. By separating fusion roller220 and release roller 268, with a cooling step therebetween via airjets 274, layer transfusion assembly 233 allows the layers to be heatedto an optimal transfusion interface temperature, and to be cooled to atemperature that fixes the layers before release. For example, thetransfusion interface temperature for transfusing the layers togethermay be at about the fusion temperature of the layer material (e.g.,about 200° C.). This substantially increases the extent to which thepolymer molecules of the transfused layers interdiffuse to promoteinterfacial entanglement, while also maintaining dimensional accuracy of3D part 222.

As shown in FIG. 6C, after passing fusion roller 220, and while buildplatform 218 remains engaged with belt 214, belt 214, build platform218, and 3D part 222 pass air jets 274. Air jets 274 may function in thesame manner as air jets 142 and 242 for cooling belt 214 the side ofrear surface 214 b. In alternative embodiments, air jets 274 may be avariety of different cooling units, such as refrigeration units,liquid-cooling units, and the like.

As discussed above, if the transfused layer 228 remains too hot,portions of it may remain adhered to front surface 214 a of belt 214,rather than cleaning releasing from belt 214. As such, cooling belt 214with air jets 274 allows the interface between front surface 214 a ofbelt 214 and the transfused layer 228 to cool so that the transfusedlayer 228 will remain adhered to 3D part 222 and cleanly release frombelt 214. This also partially assists in the active cooling of 3D part222 to maintain 3D part 222 at the average part temperature below itsdeformation temperature.

As further shown in FIG. 6D, as the transfused layer 228 passes the nipof release roller 268, belt 214 rotates around release roller 268 toseparate and disengage from build platform 218. This assists inreleasing the transfused layer 228 from belt 214, in an “assisteddelamination” step, allowing the transfused layer 228 to remain adheredto 3D part 222. Additionally, air jets 242 blow cooling air towards thetop layers of 3D part 222 as build platform 218 moves along the x-axispast release roller 268. This actively cools the transfused layer 228down to the average part temperature, as discussed above.

When build platform 218 moves 3D part 222 past release roller 268,gantry 234 may actuate build platform 218 (and 3D part 222) downward, asillustrated by arrow 278. For example, build platform 218 may beincrementally offset downward by a single layer increment. As shown inFIG. 6E, gantry 234 may then move build platform 218 (and 3D part 222)along the x-axis in the direction of arrow 280, back to a startingposition along the x-axis.

As shown in FIG. 6F, build platform 218 desirably reaches the startingposition for proper registration with the next layer 228. The sameprocess may then be repeated for each remaining layer 228 of 3D part222. Layer transfusion assembly 233 provides the benefits of transfusingthe layers together at the fusion temperature to increase interlayeradhesion, while also sufficiently cooling down the interface betweenfront surface 214 a of belt 214 and the transfused layers 228 totransfix the layers 228 in place before release from belt 214, andfurther promotes a clean release by assisting the delamination from belt214.

In addition, for each printed layer 228, the combination of air jets 242and 274 (or other cooling units) substantially removes the heat that isadded from heating elements 232, 270, 272, and from the heated fusionroller 220, prior to printing the next layer 228. This active coolingsubstantially removes the heat provided by each layer 228, therebyproviding substantially zero heat accumulation after each printed layer228. As such, 3D part 222 may be substantially maintained at an averagepart temperature that is below its deformation temperature during theentire printing operation. Further, the top layer surface temperature ofthe printed 3D part 222 may be brought back up to the fusion temperatureafter delamination using heater 270 and/or 272 of layer transfusionassembly 233 for optimal transfusion of the next layer 228.

System 10 with layer transfusion assemblies 33, 133, and 233 is suitablefor printing 3D parts (and any support structures) fromthermoplastic-based powders at high rates and with good partresolutions. In some embodiments, system 10 may print layers of a 3Dpart at a rate of at least about 40 layers per minutes (e.g., about 50layers per minute) with accurate registrations, layer thicknessesranging from about 5 micrometers to about 125 micrometers, and layerdimensions along the y-axis up to at least about 51 centimeters (about11 inches). For example, system 10 may print a 3D part at a rate ofabout three inches in height along the vertical z-axis per hour.

The resolutions of the 3D parts may also be varied based on the printingrate. For example, each 3D part may be printed at a “high quality”resolution, in which system 10 operates at a slower rate, but printswith lower layer thicknesses. Alternatively, a 3D part may be printed ata “draft quality” resolution, in which system 10 operates a faster rate,but prints with greater layer thicknesses. Furthermore, a 3D part may beprinted in “gray scale”, in which a lower density of the part materialis developed. Numerous resolutions and speeds therebetween may also beincorporated. In each of these situations, the controller may adjust theapplied pressures, temperatures, and/or contact durations during thetransfusion steps to account for the different printing rates.

System 10 is illustrated as being configured to print 3D parts (e.g., 3Dparts 22, 122, and 222) from a single thermoplastic-based powder.However, the additive manufacturing systems of the present disclosuremay also be configured to print 3D parts and/or support structures frommultiple part materials and/or support materials derived fromthermoplastic-based powders (e.g., multiple compositions and/or colors).Examples of suitable multiple-material systems include those disclosedin U.S. patent application Ser. Nos. 13/242,669 and 13/242,841.

In some embodiments, controller 24 may monitor the applied pressure, thetemperature of the layers, and the contact durations during thetransfusion steps to maximize or otherwise increase the effectiveness intransferring the layers from the front surface of the transfer belt tothe build platform. In an open-loop embodiment, one or more of theapplied pressure, temperature, and contact durations may be fixedparameters for a given part material and overall printing rate.

Alternatively, in a closed-loop embodiment, controller 24 may adjust oneor more of these parameters in response to the monitored signals usingthe one or more process control loops. For example, the controller mayadjust the pressure applied by press plate 20 or build platforms 118 and218 in response to changes in the monitored pressure and/or changes inthe monitored temperature of the layers. Moreover, controller 24 mayadjust the contract durations during the transfusion steps to compensatefor changes in the temperatures of layers and/or fluctuations in themonitored applied pressures.

As shown in FIG. 7, in some embodiments, system 10 may also include achamber 284, which can extend around layer transfusion assembly 33, andmay define an enclosable environment for printing 3D part 22. Whileillustrated in use with layer transfusion assembly 33, chamber 284 isequally suitable for use with layer transfusion assemblies 133 and 233.Chamber 284 is a temperature-controllable chamber, which providesgreater control over the active cooling step. For example, chamber 284may be maintained at the average part temperature of 3D part 22.

In these embodiments, chamber 284 may partially enclose z-axis gantry 34and belt 14, allowing z-axis gantry 34 and belt 14 to extend through thewalls of chamber 284. In alternative embodiments, heater 32 may belocated outside and upstream of chamber 284. In further alternativeembodiments, chamber 284 may be located below press plate 20, allowingbuild platform 18 to be lowered down into chamber 284. These embodimentsfurther assist in maintaining 3D part 22 at an average part temperaturethat is below its deformation temperature.

As further shown in FIG. 7, layer transfusion assembly 33, 133, or 233may also include pressure sensors (e.g., pressure sensor 286) and/orcapacitive sensors (e.g., capacitive sensor 288), each of which isconfigured to communicate with controller 24 over one or morecommunication lines (not shown). Pressure sensor 286 is one or moresensor assemblies configured to measure the transfusion pressure appliedbetween build platform 18 and press plate 20 (or between build platforms118/218 and rollers 120/220), allowing controller 24 to monitor theapplied transfusion pressure and adjust the height of build platform 18and/or press plate 20 using one or more process control loops. Examplesof suitable sensor assemblies for pressure sensor 286 include one ormore strain gauges retained on build platform 18 and/or press plate 20.

Capacitive sensor 288 is one or more capacitive sensor assembliesconfigured to measure the electrical resistance between build platform18 and press plate 20 (or between build platforms 118/218 and rollers120/220). For example, during a transfusion step, capacitive sensor 288may induce an electrical current from platen 18 to press plate 20 (orvice versa), and measure the intensity of the resulting electricalcurrent through the printed layers 28 of 3D part 22 and belt 14. Sincethe thickness of belt 14 is constant, the resulting electrical currentwill reduce as the 3D part 22 grows through the printing of successivelayers 28.

Thus, capacitive sensor 288 is suitable for monitoring the height of 3Dpart 22 and the number of layers 28 transferred to build platform 18.This allows controller 24 to accurately predict the applied pressureduring a subsequent pressing step rather than merely relying on thecalculated height of a single layer increment. This accurate predictionallows build platform 18 to be quickly raised to an intended height,rather than relying solely on feedback signals from pressure sensor 286.

Build platforms 18, 118, and 218, press plate 20, and rollers 120 and220 may each also include one or more temperature sensors (not shown)configured to respectively measure the temperatures of the buildplatforms and press plate/rollers, allowing controller 24 to hold themat the above-discussed temperatures. In a further alternativeembodiment, system 10 may include temperature sensors (not shown)configured to measure the temperatures of the 3D part layers. Forexample, system 10 may include an ultrasonic transducer for measuringthe temperature of the layer 28 retained by belt 14 and/or thetemperatures of the previously transfused layers 28 of 3D part 22 usingacoustic thermometry.

In some embodiments, controller 24 and/or host computer 26 may receiveoperational mode selections for operating system 10 in different modes.For example, a user may select operational modes such as high qualityprinting, draft quality printing, and gray scale, as discussed above.Alternatively, system 10 may receive the operational mode selections asdefault or system generated modes (e.g., a default of a high qualityprinting). These received operational mode selections may alternatively(or additionally) be set based on the geometry of the 3D part, such asif the 3D part has a small cross-sectional area and/or fine-featuredetails, as discussed above.

Upon receipt of these operational mode selections, controller 24 and/orhost computer 26 may set transfusion parameters for performing thetransfusion steps, based on the received operational mode selections.For example, the transfusion pressure, temperature(s), and or durationfor each transfusion step may be set or adjusted based on the receivedoperational mode select. This provides greater control over thetransfusion steps when operating system 10 to improve printingaccuracies and/or printing rates.

System 10 may then image a layer of the 3D part from athermoplastic-based powder (e.g., develop a layer with EP engine 12),transfer the imaged layer to a transfer medium, heat the imaged layerwhile the imaged layer is retained on the transfer medium, and transfusethe heated layer to a surface of the three-dimensional part based on theset transfusion parameters.

In some embodiments, the set transfusion parameters allow thetransfusion pressure, temperature(s), and or duration for eachtransfusion steps to vary between different transfusion steps. Forexample, if a first portion of a 3D part contains a simple blockgeometry and a second portion of the 3D part contains a fine-featuregeometry, controller 24 and/or host computer 26 may set the transfusionparameters such that the layers used to form the simple block geometryare transfused differently (e.g., higher transfusion pressure) fromthose used to form the fine-feature geometry (e.g., lower transfusionpressure).

FIGS. 8 and 9A-9C illustrate a suitable embodiment for belt 14 (shown inFIGS. 1, 2, and 4A-4D), and is equally suitable for belt 114 (shown inFIGS. 5A-5E) and belt 214 (shown in FIGS. 6A-6F). In the embodimentshown in FIG. 8, belt 14 is a multiple-layer belt that includes layer orfilm 290 (defining front surface 14 a) and base portion 292 (definingrear surface 14 b).

Film 290 and base portion 292 are desirably derived from materials thatare suitable for transferring the layers 28 of part (or support)materials from EP engine 12 to build platform 18, that are thermallystable at the fusion temperatures of the part and support materials, andthat are robust for continued operation at high rotational speeds whilebeing repeatedly heated and cooled during the heating and active coolingsteps.

Film 290 is derived from one or more low-surface energy materials,thereby allowing the received layers 28 to effectively release fromfront surface 14 a to build platform 18. Examples of suitable materialsfor film 290 include one or more fluorinated polymers, such aspolytetrafluoroethylenes (PTFE), fluorinated ethylene propylenes, andperfluoroalkoxy polymers. Examples of suitable commercially availablefluorinated polymers include PTFE available under the trade designation“TEFLON” from E.I. du Pont de Nemours and Company, Wilmington, Del.

Base portion 292 is derived from one or more materials that promote goodelectrostatic attraction for the thermoplastic-based powders to frontsurface 14 a via triboelectric charges. Examples of suitable materialsfor base portion 292 include one or more polyimide materials, such asthose commercially available under the trade designation “KAPTON” fromE.I. du Pont de Nemours and Company, Wilmington, Del., which may bedoped with one or more conductive materials to promote the triboelectriccharges. In some embodiments, belt 14 may also include one or moreadditional layers between film 290 and base portion 292, such as one ormore tie layers.

FIGS. 9A-9C illustrate alternative embodiments for belt 14 for engagingwith various drive rollers of system 10. As shown in FIG. 9A, frontsurface 14 a of belt 14 may include receiving region 294 and edgeregions 296 on opposing lateral sides of receiving region 294. Receivingregion 294 is the region of front surface 14 a on which layers 28 areretained for transfer between EP engine 12 and build platform 18. Edgesregions 296 are the regions at which one or more drive mechanisms mayengage drive belt 14.

For example, one or more rollers (e.g., rollers 16, nip roller 120,fusion roller 220, release roller 268, and/or any service-loop roller)may engage front surface 14 a and/or rear surface 14 b at edge regions296 to ensure the rollers to not interfere with the developed layers 28.In some embodiments, pairs of opposing rollers (not shown) maysimultaneously engage front surface 14 a and rear surface 14 b at edgeregions 296 to nip and drive belt 14 in the direction of arrow 30.

Registration along the x-axis may be maintained with the use of encodermarkings 298. Encoder markings 298 may be pre-marked on front surface 14a and/or rear surface 14 b at preset increments along the x-axis, or maybe printed with the developed layers 28 to identify relative locationsof the developed layers 28 along the x-axis. System 10 may also includeone or more optical readers (e.g., optical reader 299) to locate encodermarkings 298 as belt 14 rotates in the direction of arrow 30.

Alternatively, as shown in FIG. 9B, belt 14 may include an array ofholes 300 or other openings that extend through film 290 and baseportion 292 adjacent to the lateral edges of belt 14. Holes 300 areconfigured to engage with reciprocating gear teeth (not shown) of one ormore rollers (e.g., rollers 16, nip roller 120, fusion roller 220,release roller 268, and/or any service-loop roller) to drive belt 14 ina tractor-feed manner. In this embodiment, registration along the x-axismay also be maintained with the use of encoder markings 298, if desired.Alternatively, holes 300 may themselves function as encoder markings inthe same manner. System 10 may also include one or more optical readers(e.g., optical reader 299) to locate encoder markings 298 and/or holes300 as belt 14 rotates in the direction of arrow 30.

FIG. 9C shows yet another alternative embodiment in which belt 14includes rear ribs 302 that extend laterally along rear surface 14 b.Ribs 302 are configured to engage with reciprocating gear teeth (notshown) of one or more rollers (e.g., rollers 16, nip roller 120, fusionroller 220, release roller 268, and/or any service-loop roller) to drivebelt 14 in a timing-belt manner. In this embodiment, registration alongthe x-axis may also be maintained with the use of encoder markingscorresponding to encoder markings 298, if desired. Alternatively, ribs300 may themselves function as encoder markings in the same manner.System 10 may also include one or more optical readers (e.g., opticalreader 299) to locate the encoder markings and/or holes ribs as belt 14rotates in the direction of arrow 30.

FIGS. 9A-9C illustrate example engagement mechanisms for belt 14,allowing belt 14 to engage with one or more drive mechanisms of system10. However, belt 14 may alternatively include different engagementmechanisms as particular designs may require.

FIGS. 10-12 are flow diagrams of embodied methods for the layer transfertechnique of the present disclosure, which may be performed with system10. FIG. 10 illustrates method 310, which may be performed with system10 having layer transfusion assembly 33 (shown in FIGS. 1, 2, and 4A-4D)and/or layer transfusion assembly 133 (shown in FIGS. 5A-5E). As shown,method 310 includes step 312-324, and initially involves developing orotherwise imaging a layer (step 312), such as with EP engine 12. Theimaged layer may then be transferred on a transfer medium (e.g., belts14 and 114) from a first location at EP engine 12 to a second locationat the layer transfusion assembly (e.g., layer transfusion assemblies 33and 133) (step 314).

Prior to reaching the second location at the layer transfusion assembly,the layer is heated to at least a fusion temperature of thethermoplastic-based powder (e.g., at heaters 32 and 132) (step 316).Upon reaching the layer transfusion assembly, the heated layer is thentransfused and transfixed in a combined step (step 318).

For example, for layer transfusion assembly 33, press plate 20 mayengage build platform 18 to transfuse the heated layer 28 to the topsurface of 3D part 22. Because press plate 20 may be heated to atemperature that is lower than the fusion temperature, the contactbetween press plate 20 and rear surface 14 b of belt 14 cools down theinterface between belt 14 and the transfused layer 28, increasing theadhesive force of the interdiffused polymers in the transfused layer 28and 3D part 22 relative to the adhesive force of the transfused layer 28to surface 14 a of belt 14, thereby maintaining the transfused layeradhered to the 3D part in a fixed spatial position.

The transfused and transfixed layer may then be released from thetransfer medium (step 320), such as by retracting press plate 20 and/orbuild platform 18, or by the separation of belt 114 from build platform118 by belt 114 winding around nip roller 120. The transfixing stepdiscussed above allows the transfused layer to cleanly release from thetransfer medium and remain adhered to the 3D part.

The 3D part may then be actively cooled (e.g., with air knives 42 andair jets 142) (step 322). As discussed above, because the imaging system(e.g., system 10) is capable of printing the layers at speeds that aremuch faster than the rate at which heat diffuses through the variablethermal resistance of the 3D parts, heat can accumulate in the 3D parts,which, if not accounted for, can exceed a deformation temperature of the3D part, causing the bulk of the 3D part to soften enough reduces itsstructural integrity. Such a soft part may deform under asubsequently-applied transfusion pressure during a subsequenttransfusion step.

To overcome this issue while maintaining fast printing rates, the 3Dpart may be actively cooled between each transfusion step 318 tomaintain the 3D part at an average part temperature that is lower thanthe deformation temperature of the 3D part. Steps 312-324 may then berepeated for each layer of the 3D part until the printing operation iscompleted (as indicated by arrow 324). By heating each layer to at leastthe fusion temperature of the thermoplastic-based powder, followed bytransfusing/transfixing, and active cooling allows system 10 to print 3Dparts with good part quality and strengths (e.g., z-strengths).

FIG. 11 illustrates method 326, which is similar to method 310 (shown inFIG. 10), and may be performed with system 10 having layer transfusionassembly 233 (shown in FIGS. 6A-6F) (and with layer transfusion assembly33 in the embodiment in which belt 14 remains in contact with thetransfused layer 28 after press plate 20 retracts). Method 326 includessteps 328-342, where steps 328, 330, 332, 338, 340, and 342 may beperformed in the same manner as the respective steps of method 310.

However, instead of a combined transfusion and transfixing step 318 ofmethod 310, method 326 includes a transfusion step 334 and transfixingstep 335, which are separate. for example, layer transfusion assembly233 includes a heated fusion roller 220 (for transfusion step 334) andrelease roller 268 (for release step 338), which are separated by airjets 274 (for a cooling or transfixing step 336). This allows the layersto be heated to an optimal transfusion interface temperature at theheating step 332, and during the transfusion step 334, and then to becooled to a temperature that fixes the layers (at transfixing step 336)before release at release step 338. This substantially increases theextent to which the polymer molecules of the transfused layersinterdiffuse to promote interfacial entanglement, while also maintainingdimensional accuracy of the 3D part.

Moreover, the release of the transfused layer from the transfer mediummay be assisted during the release step 338. For example, release roller268 may assist in releasing transfused layer 228 from belt 214 byincreasing the angle of separation between belt 214 and build platform218, which increases the ease at which transfused layer 228 delaminatesfrom belt 214.

As further shown in FIG. 11, the active cooling step 340 may be anoptional step of method 326 (as illustrated with the broken lines 344).For example, system 10 may instead operated at a lower printing speed toallow heat to diffuse from the 3D part. However, as discussed above, theactive cooling step 340 is desirable for maintaining the structuralintegrity of the 3D part while printing at high speeds.

FIG. 12 illustrates method 346, which is similar to method 310 (shown inFIG. 10) and method 326 (shown in FIG. 11), and may be performed withsystem 10 having layer transfusion assembly 233 (shown in FIGS. 6A-6F).Method 346 may also be performed with layer transfusion assembly 33 inthe embodiment that includes one or more heaters corresponding toheaters 270 and 272. Method 346 includes steps 348-364, where steps 348,350, 352, 356, 358, 360, 362, and 364 may be performed in the samemanner as the respective steps of method 326.

However, method 346 also includes step 354, in which the top surface orlayer(s) of the 3D part is also pre-heated prior to the transfusion step356. For example, with layer transfusion assembly 233, heaters 270 and272 may heat the top surface or layer(s) of 3D part 222 to at least thefusion temperature of the thermoplastic-based powder. Because layer 228and the heated top surface/layer of 3D part 222 are each heated to thefusion temperature of the layer material, the pressed heated layer 228transfuses to the heated top surface/layer of 3D part 222 with a highlevel of interlayer adhesion (during transfusion step 358).

Furthermore, by separating fusion roller 220 and release roller 268,with a cooling or transfixing step 358 therebetween via air jets 274,layer transfusion assembly 233 allows the layers to be heated to anoptimal transfusion interface temperature, and to be cooled to atemperature that fixes the layers before release. This substantiallyincreases the extent to which the polymer molecules of the transfusedlayers interdiffuse to promote interfacial entanglement, while alsomaintaining dimensional accuracy of the 3D part.

EXAMPLES

The present disclosure is more particularly described in the followingexamples that are intended as illustrations only, since numerousmodifications and variations within the scope of the present disclosurewill be apparent to those skilled in the art.

Example 1

For the printing operations of Example 1, 3D parts were printed with anadditive manufacturing system corresponding to the system shown in FIGS.1, 2, and 4A-4D, having a moveable press plate and avertically-actuatable build platform, each of which were heated. The 3Dparts were printed from an ABS part material, where each layer wasdeveloped with an electrophotography engine and transferred to arotatable transfer belt looped around a service loop design. Thedeveloped layer was carried by the transfer belt to an infrared heaterto heat the layer to a fusion temperature of ABS part material.

The heated layer was then transferred to and aligned over the buildplatform. The press plate was actuated downward to transfuse the heatedlayer against a previously-transfused layer of the 3D part (or to thebuild platform for the initial layer). The press plate was thenretracted, which cleanly delaminated the layer from the transfer belt,allowing the layer to remain transfused to the 3D part.

Cooling air from an air knife was also directed to the top layers of the3D part. This removed the additional heat from the transfused layer toprovide an average part temperature of that maintained the structuralintegrity of the 3D part, where the 3D part did not slump or melt due toheat accumulation. This process was repeated for each layer of the 3Dpart. The air knife and the press plate successfully maintained theaverage part temperature below the deformation temperature of the 3Dpart during the entire printing operation. The resulting 3D partsexhibited good part resolution, dense fill, and good dimensionalintegrity.

Example 2

For the printing operations of Example 2, 3D parts were printed with anadditive manufacturing system corresponding to the system shown in FIGS.5A-5E, having a nip roller and a moveable build platform, each of whichwere heated. The 3D parts were printed from an ABS part material, whereeach layer was developed with an electrophotography engine andtransferred to a rotatable transfer belt looped around a service loopdesign. In these Examples, the service loop was used to protect thetransfer belt against tension stress. The developed layer was carried bythe transfer belt to an infrared heater to heat the layer to a fusiontemperature of the ABS part material.

The heated layer was then transferred to and aligned over the buildplatform. The build platform was actuated upward to transfused theheated layer against a previously-transfused layer of the 3D part (or tothe build platform for the initial layer). In particular, actuating thebuild platform upward pressed the heated layer and transfer belt betweenthe build platform (or against a previously-transfused layer of the 3Dpart) and the nip roller. The build platform was then moved at asynchronized rate with the transfer belt, and then released at adownstream location. This cleanly delaminated the layer from thetransfer belt, allowing the layer to remain transfused to the 3D part.

Cooling air from air lets was also directed to the top layers of the 3Dpart. This removed the additional heat from the transfused layer toprovide an average part temperature of that maintained the structuralintegrity of the 3D part, where the 3D part did not slump or melt due toheat accumulation. The build platform was then moved back to itsstarting position, and the process was repeated for each layer of the 3Dpart. In these Examples, the air jets also successfully maintained theaverage part temperature below the deformation temperature of the 3Dpart during the entire printing operation. The resulting 3D parts alsoexhibited good part resolution, dense fill, and good dimensionalintegrity.

Comparative Examples A and B

For the printing operations of Comparative Examples A and B, 3D partswere printed with the same additive manufacturing systems respectivelyused for Examples 1 and 2. However, for Comparative Examples A and B,the air knife or jet cooling was omitted. Otherwise, the processes wereperformed in the same manner as discussed above for Examples 1 and 2.

For the printing operations of Comparative Examples A and B, prior tothe completion of each printed 3D part, the printed layers began tocompress and flatten. As discussed above, this is believed to be due toheat accumulating in the printed layers, which was unable to diffusesufficiently between each printed layer. The accumulated heat softenedthe bulk of the 3D part, causing it to compress during subsequenttransfusion steps. This resulted in deformed 3D parts.

As such, the layer transfer technique of the present disclosureincluding the active cooling is beneficial for printing 3D parts at highrates using electrophotography. The active cooling was successfullyimplemented to remove the added heat from each fused layer prior to thetransfusion of the next layer. This allowed the 3D parts printed withthe systems in Examples 1 and 2 to be maintained at average parttemperatures below their deformation temperatures, but high enough topromote good interlayer adhesion and reduced curl.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

The invention claimed is:
 1. An additive manufacturing system forprinting a thermoplastic part, the additive manufacturing systemcomprising: an imaging engine configured to develop imaged layers of athermoplastic-based powder having a fusion temperature; a movable buildplatform; a transfer medium having a transfer surface configured tosequentially receive and convey the imaged layers from the imagingengine to the build platform, and having an opposing contact surface; aheater configured to heat the imaged layers on the transfer medium to atleast the fusion temperature; a heater configured to heat a previouslytransfused layer of a thermoplastic part being printed on the buildplatform to at least the fusion temperature; a heated layer transfusionelement configured to transfuse a heated imaged layer conveyed by thetransfer medium onto the heated previously fused layer by engaging thecontact surface of the transfer medium so that the transfer medium andthe heated imaged layer are pressed between the transfuse element andthe build platform, and to disengage therefrom without releasing thetransfer medium from the transfused layer; and a cooling unit configuredto actively cool the transfused layer to below the fusion temperaturewhile it remains on the transfer medium, so as to transfix thetransfused layer before delaminating it from the transfer medium.
 2. Theadditive manufacturing system of claim 1, wherein the layer transfusionelement is heated to the fusion temperature.
 3. The additivemanufacturing system of claim 1, wherein the layer transfusion elementcomprises a press plate.
 4. The additive manufacturing system of claim3, wherein the moveable build platform is configured to release thetransfer medium from the transfused layer by moving away from thetransfer medium.
 5. The additive manufacturing system of claim 1,wherein the transfer medium comprises a rotatable belt, and wherein therotatable belt revolves in a rotational path from the imagine engine,past the heater, in between the layer transfusion element and buildplatform, and past the cooling unit.
 6. The additive manufacturingsystem of claim 1, wherein the layer transfusion element comprises afusion roller.
 7. The additive manufacturing system of claim 6, whereinthe transfer medium comprises a rotatable belt that revolves in arotational path from the imagine engine, past the heater, in between thelayer transfusion element and build platform, and past the cooling unit,and wherein the moveable built platform is configured to move at asynchronized rate and direction with the rotatable belt while the fusionroller presses the heated layer to previously fused layers of theprinted three-dimensional part.
 8. The additive manufacturing system ofclaim 7, and further comprising a release roller disposed downstream ofthe cooling unit in the rotational path of the rotatable belt andconfigured to engage the contact surface of the rotatable belt so as toassist in delaminating the transfused layer from the transfer mediumafter it is transfixed.
 9. The additive manufacturing system of claim 1,wherein the heater comprises a non-contact radiant heater.
 10. Anadditive manufacturing system for printing a thermoplastic part, theadditive manufacturing system comprising: an imaging engine configuredto develop imaged layers of a thermoplastic-based powder having a fusiontemperature; a movable build platform; a transfer medium having atransfer surface configured to sequentially receive and convey theimaged layers from the imaging engine to the build platform, and havingan opposing contact surface; a heater configured to heat the imagedlayers on the transfer medium to at least the fusion temperature; aheater configured to heat a previously transfused layer of athermoplastic part being printed on the build platform to at least thefusion temperature; a heated fusion roller configured to be heated to atransfer temperature and further configured to transfuse a heated imagedlayer conveyed by the transfer medium onto the heated previously fusedlayer by engaging and rolling across the contact surface of the transfermedium, and to disengage therefrom without releasing the transfer mediumfrom the transfused layer; a cooling unit configured to actively coolthe transfused layer to below the fusion temperature while it remains onthe transfer medium so as to transfix the transfused layer; and arelease roller disposed downstream of the cooling unit and configured toengage the contact surface of the transfer medium so as to assist indelaminating the transfused layer from the transfer medium.
 11. Theadditive manufacturing system of claim 10, wherein the transfer mediumis a rotatable belt that follows a rotational path from the imagineengine, past the heater configured to heat the imaged layers, in betweenthe fusion roller and build platform, past the cooling unit and therelease roller, and returning to the imaging engine.
 12. The additivemanufacturing system of claim 11, wherein the moveable built platform isconfigured to move at a synchronized rate and direction with therotatable belt while the fusion roller is engaged to transfuse a heatedimaged layer.
 13. The additive manufacturing system of claim 10, whereinone or more heaters comprise a non-contact radiant heater.
 14. Theadditive manufacturing system of claim 10, wherein the cooling unitcomprises at least one air knife, at least one air jet, or a combinationthereof.
 15. A method for printing a three-dimensional part with anadditive manufacturing system, the method comprising: imaging a layer ofthe three-dimensional part from a thermoplastic-based powder;transferring the imaged layer to a transfer medium; heating the imagedlayer while the imaged layer is retained on the transfer medium; heatinga top surface of the three-dimensional part; transfusing the heatedlayer to the heated top surface of the three-dimensional part; coolingthe transfer medium and at least a portion of the transfused layer; andreleasing the transfused layer from the cooled transfer medium such thatthe transfused layer remains adhered to the three-dimensional part. 16.The method of claim 15, wherein imaging the layer comprises developingthe layer with an electrophotography engine.
 17. The method of claim 15,and further comprising further cooling the transfused layer to hold theprinted three-dimensional part at about an average part temperature thatis below a deformation temperature of the three-dimensional part. 18.The method of claim 15, wherein the transfer medium comprises arotatable belt, and wherein the method further comprises: rotating therotatable belt at a rotational rate; and moving the three-dimensionalpart at a rate that is synchronized with the rotational rate of therotatable belt.
 19. The method of claim 15, wherein heating the imagedlayer comprises heating the imaged layer to at least a fusiontemperature of the thermoplastic-based powder, and wherein heating thetop surface of the three-dimensional part comprises heating the topsurface of the three-dimensional part to at least the fusion temperatureof the thermoplastic-based powder.
 20. The method of claim 15, whereinthe transfused layer defines a new top surface of the three-dimensionalpart, and wherein the method further comprises: imaging a subsequentlayer of the three-dimensional part from the thermoplastic-based powder;transferring the subsequent imaged layer to the transfer medium; heatingthe subsequent imaged layer while the subsequent imaged layer isretained on the transfer medium; heating the new top surface of thethree-dimensional part; and transfusing the heated subsequent layer tothe heated new top surface of the three-dimensional part.